The present invention relates generally to remote reporting systems for elements in digital transmission line systems and, more particularly, to a digital line element such as a repeater or network interface unit (NIU) that may remotely report its status (such as its operating mode or location) to a remote facility interconnected to a digital transmission line. In this way, for example, the invention may assist a telephone company technician in identifying, from a test location, the location of a malfunctioning line element among a string of line elements such as multiple repeaters and an NIU.
The present invention may be used with digital transmission lines generally, including, for example, the Bell Telephone System in the United States. The data, or "payload", signals on such transmission lines are typically sent differentially on a Tip-Ring pair. Payload signals are received by the telephone company central office, and generally, are transmitted, via cables, to a series of regenerative signal repeaters and an NIU in a T1 span line. Such repeaters are spaced along the cables approximately every 6,000 feet. The first repeater receives the data from the central office, but, because of transmission line losses, noise, interference, and distortion, the signal will have degenerated. The repeater recognizes the presence or absence of a pulse at a particular point in time and thereafter, if appropriate, regenerates, or "builds up," a clean, new pulse. The first line repeater (or "signal repeater" or "regenerative repeater") sends the regenerated, or repeated, signal to the next line repeater, stationed approximately one mile away. The last repeater then transmits its pulses to an NIU at the remote end of the span line typically located at the point of demarcation between the network and the customer premises.
The Bell Telephone System has widely utilized time multiplexed pulse code modulation systems. Such systems have generally been designated as "T carriers." The first generation of multiplexers designed to feed the T1 system was the D1 channel bank. Channel banks have evolved through the D5 series. The "D" channel bank provides multiple DS-1 signals that are carried on the T1 systems. Each T1 system carries twenty-four two-way channels on two pairs of exchange grade cables. One pair of cables provides communication in each direction.
The data to be transmitted over the cables, such as speech, may be sampled at a rate of 8,000 hertz, and the amplitude of each signal is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into a binary form. Representative binary pulses appear on the transmission lines.
The binary form of each sample pulse consists of a combination of seven pulses, or bits. An eighth bit is added to the end of the combination, or byte, to allow for signaling.
Each of the twenty-four channels on the T1 system is sampled within a 125 microsecond period (equivalent to 1/8,000 of a second). The period is called a "frame." Within each frame, an additional, synchronizing bit is added in order to signal the end of a frame. Otherwise, a single error might cause future representations of the data on the transmission line to be misunderstood by the receiving apparatus.
Since there are eight bits per channel and there are twenty-four channels, and there is one pulse at the end of each frame, the total number of "bits" needed per frame is 193. Thus, the resulting line bit rate for T1 systems is 1.544 million bits per second.
A coding system is typically used to convert the analog signal to a digital signal. The system guarantees some desired properties of the signal, regardless of the pattern to be transmitted. The most prevalent code in the United States is bipolar coding with an all zero limitation (also called "AMI" for Alternative Mark Inversion).
With bipolar coding, alternate "ones" are transmitted as alternating positive and negative pulses, assuring a direct current balance and avoiding base-line wander. Further, an average density of one pulse in eight slots, with a maximum of fifteen zeros between "ones," is required. This is readily obtained in voice-band coding, however, by simply not utilizing an all-zero word.
Another arrangement, also used to guarantee density with a bipolar code, replaces strings of zeros with two successive pulses of the same polarity, allowing its identification and removal at the receiving end. This arrangement, called B8ZS (for Bipolar with 8-Zero Substitution) is also in considerable use. Other coding arrangements (such as B3ZS and 4B3T) have also been established.
Signals that violate the rules established in a particular system are detected as types of errors. Thus, for example, under a bipolar coding scheme, two positive pulses should never occur in sequence (except in B8ZS encoded all-zeros). To the extent such pulses do occur adjacent to each other, such a signal may be noted as a bipolar violation. Test sets applied to digital transmission cables may detect the number of bipolar violations over a predetermined period of time to test the operational integrity of the transmission lines.
There may be many miles of cable between the central office and the customer premises, with a large number of repeaters between the two facilities. Thus, if the malfunction of a transmission line is detected during a test (or simply during normal operation), it is important to make an accurate determination of the location of the fault. In this way, the fault may be located and corrected more quickly and inexpensively.
Furthermore, to assist in the testing of transmission lines and correction of faults, a technician may wish a repeater, an NIU or another transmission line element to identify not only its location with respect to the test set, but also (or alternatively) its condition. For example, a repeater may be able to enter a particular operating mode, such as "logical loop back" or "metallic loop back," and a technician may command the repeater to communicate back to the technician that it is in such a mode.
Also, for example, if a repeater detects that an adjacent repeater or span of transmission cable is not functioning, the repeater may move to an open power loop mode, signifying that the adjacent span or repeater is malfunctioning. The repeater that is reporting the malfunction should be able to identify itself to a testing technician so that the technician may more readily locate the fault.
Further, a repeater may, for example, be in a loop back mode as result of commands issued by a first technician. A second technician may wish to know which repeater has been placed in the loop back mode. The repeater in the loop back mode should be able to identify its location and condition to the second technician, such that more efficient testing and repair of the transmission lines may be effected.
Still further, for example, a technician positioned remotely from an NIU may wish the NIU to report on the status or condition(s) of the network transmission lines and/or the conditions of the customer premises equipment and customer transmission lines. In response, the NIU should be able to communicate such information to the requesting technician.
Unfortunately, many of the presently available apparatus and methods for communicating with transmission line elements, such as repeaters and NIUs, are cumbersome and expensive to manufacture. The presently available reporting systems often substantially increase the size, weight, and complexity of the line elements. Moreover, such systems may involve the use of specialized codes, such that the technicians must utilize modified line elements as well as new, specially designed test equipment in order to allow the test equipment and line elements to communicate with each other.